The King waved to well-wishers as he attended the traditional Easter Sunday church service at Windsor Castle in a rare public appearance since he was diagnosed with cancer.
The 75-year-old monarch, who has stepped back from major public engagements while receiving treatment, was joined by the Queen and other members of the Royal Family.
But the Prince and Princess of Wales were not at the service, coming just over a week after Kate released an emotional video message revealing she had started preventative chemotherapy for cancer.
Charles, wearing a dark coat and blue tie, smiled and waved at members of the public as he arrived at St George’s Chapel.
One person shouted “Happy Easter”, to which Charles gestured with his arm and responded: “And to you.”
The Princess Royal and her husband Vice Admiral Sir Timothy Laurence, the Duke and Duchess of Edinburgh and the Duke of York and Sarah, Duchess of York, all arrived at the chapel a few moments earlier.
Kate, 42, William, 41, and their three children – George, 10, Charlotte, eight, and five-year-old Louis – are spending the Easter holidays together as they adjust to her diagnosis, which was discovered in post-operative tests after abdominal surgery.
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The King’s attendance at church will be seen as a move to reassure the public after the shock news about his daughter-in-law.
Kate’s video message followed weeks of speculation and conspiracy theories on social media about her whereabouts because she had not been seen on official engagements since last December.
Speculation was fuelled further when she admitted editing a family photograph posted on Mother’s Day.
The King, who was diagnosed with cancer in February, is said to be “proud” of his “beloved” daughter-in-law for her “courage” in speaking about her treatment, while Harry and Meghan have also offered words of support.
Earlier this month, Charles said he had been reduced to tears by the messages and cards of support he had received.